This project is aimed to establish a genetic basis of male recombination-mutator (Mr) system of Drosophila melanogaster and to understand its role in evolution. The following specific points will be studied intensively. 1. There is evidence now available to indicate that the Mr chromosomes segregate in frequencies larger than the theoretical 0.5 in nature. Such chromosomes will be analyzed with respect to the elements of Segregation Distorter (SD) system in Drosophila. 2. Under specific genotype conditions, recombination was found to occur in heterochromatin in females. This unusual phenomenon appears to be associated with so-called hybrid dysgenesis. There seems to be segregation distortion in females when she is inducing heterochromatic recombination. These points will be established. 3. In some special cases, F1 female sterility (hybrid dysgenesis) seems to occur even if the genotypes (and cytotypes) of parents are the same. This point will be critically examined.